1. Field of the Invention
This invention relates to zoom lenses of the rear focus type and, more particularly, to high range, large relative aperture zoom lenses of the rear focus type having a range of 8 to 10 and F-number of about 1.8 and having a short total length, to be used in photographic cameras, video cameras, or broadcast cameras.
2. Description of the Related Art
It has been known to make the focusing provision in a lens unit other than the front or first lens unit of a zoom lens for a photographic camera or video camera. The so-called rear focus type, among others, has found its use in an increasing number of zoom lenses.
The advantages that the zoom lenses of the rear focus type of zoom lens, generally have as compared with an ordinary type which, when focusing, moves the first lens unit, are that the effective diameter of the first lens unit gets small, making it easier to minimize the bulk and size of the entire lens system, and that close-up photography, particularly macrophotography, becomes easy to perform, and further that, since the focusing lens unit is of relatively small size and light weight, a weaker driving power for that lens unit is sufficient, thereby allowing focusing to be performed quickly and easily.
As examples of such a rear focus type zoom lens, mention may be made of Japanese Laid-Open Patent Application No. Sho 63-44614, in which, where, as counted from the object side, a first lens unit of positive refractive power, a second lens unit of negative refractive power for varying the focal length, a third lens unit of negative refractive power for compensating for the image shift with zooming and a fourth lens unit of positive refractive power, totaling four lens units, constitute a so-called 4-unit zoom lens wherein the third lens unit is moved to effect focusing. This feature has to secure a space for movement of the third lens unit. Accordingly, the physical length tends to increase.
In Japanese Laid-Open Patent Application No. Sho 58-136012, the zooming section is constructed with three or more lens units, one of which is made movable for focusing.
Japanese Laid-Open Patent Applications Nos. Sho 62-24213 and Sho 63-247316 disclose a zoom lens having four lens units of which the first, when counted from the object side, is of positive refractive power, the second is of negative refractive power, the third is of positive refractive power and the fourth is of positive refractive power, wherein the second lens unit is moved to vary the focal length and the fourth lens unit is moved to compensate for the image shift with zooming and is given the focusing function.
In Japanese Laid-Open Patent Application No. Sho 58-160913, a first lens unit of positive power, a second lens unit of negative refractive power, a third lens unit of positive refractive power and a fourth lens unit of positive refractive power are arranged in this order from the object side, wherein the first and second lens units are moved to vary the focal length, while the fourth lens unit is moved to compensate for the image shift with zooming, whereby one or two or more of these lens units are moved to effect focusing.
Recently, in the field of art of video cameras, an image sensing means in the form of a solid-state image sensor (CCD) has been reduced in size. In more detail, a solid-state image sensor of as small a size as 1/3 or 1/4 in. is used in place of one of conventional size, or 2/3 or 1/2 in. With the reduction in size of the solid-state image sensor, there is a demand for even more of a reduction in the size of the zoom lens which is to be used therewith.
With respect to the photographic lens for use in the video camera, the distance from the last lens surface to the image sensing plane, i.e., the back focal distance, is relatively long so that the dust or foreign particles on the last lens surface do not have a bad influence on the image projected on the image sensing plane.
If, as the design of the zoom lens adapted to, for example, the 1/2 in. image sensor is used in making up a zoom lens for the 1/4 in. image sensor, the dimensions of the zoom lens are reduced merely in proportion, however, the back focal distance, too, gets proportionally (1/2 times) shorter. Then, the foreign particles on the last lens surface cast their appreciable shadow on the image sensing plane of the image sensor, giving rise to a problem that the image quality is lowered. For this reason, the zoom lens for a video camera has, despite the reduction of the size of the image sensor, to have its back focal distance kept longer than a predetermined value.
In general, the use of the rear focus type in the zoom lenses leads to possibilities of minimizing the bulk and size of the entire lens system and also of speeding up the focusing process.
In turn, however, the variation of aberrations with focusing is caused to increase, giving rise to a problem in that the optical performance is very difficult to keep high throughout the entire range of object distances while still permitting minimization of the size of the entire lens system to be achieved. Particularly, for a large relative aperture and high range zoom lens, another problem arises in that good stability of optical performance is very difficult to obtain throughout the entire zooming range as well as the entire focusing range.